Newly developed procedures for isolation of tissue activator of plasminogen from pig heart have made available highly purified tissue activator preparations with specific activities of 100,000-200,000 CTA U/mg protein, equivalent to the best human urokinase preparations. As antisera against pig heart activator has been used to show immunological identity between tissue activator and circulating blood activator. Other experimental evidence indicates pig heart activator is obtained primarily as the precursor form. Plasminogen activators from other pig organs and plasma euglobulin will be isolated to determine if their physical, enzymatic and immunological properties, clot lysis and fibrin plate activities, ability to activate purified plasminogens from human, bovine and pig plasmas, immunodiffusion and innunoelectrophoretic properties, as well as esterase activity changes will be observed and correlated with the study of mechanisms for the conversion of the precursor to the active activator form. Other structure-function relationships will be determined by measuring the effects of active site inhibitors, sulfhydryl blocking agents and disulfide compounds. Purification of human tissue activator, employing the purification methods already developed for pig heart, will continue. A primary objective will be to produce an antiserum against human tissue activator which has cross-reactivity with circulating blood activator. This would allow development of an immunochemical assay to investigate the role of tissue activator in localized and systemic fibrinolysis in the maintenance of health and in disease processes.